A semiconductor laser comprising AlGaInP has the shortest oscillation wavelength among III-V group compound semiconductors. The operating wavelength is 0.63 microns. This laser draws attention as one which can be used in place of a helium neon laser. In a production of a semiconductor laser, Zn or the like is diffused into a semiconductor layer so as to change the refractive index or the conductivity type thereof.
FIG. 4 shows a cross-sectional view of a prior art method of diffusing Zn into AlGaInP.
In FIG. 4, reference numeral 1 designates a GaAs substrate. An Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P layer 2 is deposited on the GaAs substrate 1 by MOCVD or MBE. A Si.sub.3 N.sub.4 diffusion mask 4 is deposited on a portion of the Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P layer 2 by CVD. A mixed layer of ZnO and SiO.sub.2 5 is deposited on the Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P layer 2 and the diffusion mask 4 to a thickness of 1500 Angstroms by sputtering method. Herein, the weight ratio of ZnO to SiO.sub.2 is 9:1. A SiO.sub.2 protection layer 6 is deposited on the mixed layer 5 to a thickness of 1000 Angstroms by sputtering. Reference numeral 7 designates a Zn diffusion region.
The diffusion process will be described.
A sample obtained by depositing the Si.sub.3 N.sub.4 diffusion mask 4, ZnO:SiO.sub.2 mixture layer 5, and the SiO.sub.2 protection layer 6 on the Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P layer 2 is set in a diffusion furnace, and annealed at a temperature of 570.degree. C. in a nitrogen or hydrogen ambient atmosphere for an hour. The Zn of the ZnO:SiO.sub.2 mixed layer 5 is diffused into the Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P layer 2 to a depth of about 1.3 microns. In order to precisely control the diffusion depth, it is necessary to precisely control the diffusion temperature and time. Further, in order to control the concentration of the diffusion region, it is also necessary to precisely control the temperature and time.
In this prior art diffusion method, the diffusion depth and the concentration of the diffusion region cannot be precisely controlled without precisely controlling the diffusion time and temperature. When the diffusion is actually conducted, variations in film thickness and the composition of the AlGaInP layer on one wafer and variations in AlGaInP layer thicknesses on different wafers need to be considered. It is difficult to place the diffusion front repeatedly at a position of predetermined depth in the entire wafer. Furthermore, when the temperature is set lower in order to reduce the diffusion speed, the diffusion time can be easily controlled, but the Zn concentration in the diffusion region is lowered, thereby adversely affecting the device characteristics.
Furthermore, the Si.sub.3 N.sub.4 generally employed as a diffusion mask, is produced by sputtering in an apparatus different from that in which the semiconductor layers are grown, thereby complicating the production process.